1. Field of the Invention
This invention relates to a refiner, namely, a machine that gives mechanical treatment to wood chips and fiber for paper stock, which refiner is vented to release steam from the refining zone, and to the process of venting the refining zone of a refiner.
2. The Problem
In refiners, pulp fibers are shredded between closely spaced disks having shredding surfaces with generally radial ribs forming approximately radial grooves between them. The relative motion of the ribs rolls and tears the fiber material and the material is progressively refined into pulp. The working of the fiber material will be referred to as shredding effected by fiber to rib impacts, friction between the pulp and the ribs and rubbing of fiber on fiber.
In order to maximize the shredding action of the ribs, the coacting ribbed disks are placed close together to form a thin refining zone between them, but the friction and shredding of the fibers between the refining disks produces a large amount of heat. The pulp fibers usually have a high water content (30 percent to 90 percent by weight) and, in addition, water may be added to the fiber mass to avoid excessively high temperatures and to optimize the fiber friction. The temperatures produced, however, are sufficiently high to vaporize water and generate great amounts of steam in the refining zone, as discussed in Hellerqvist U.S. Pat. No. 4,221,631, issued Sep. 9, 1980, at column 1, lines 27 to 30 as follows:
From this water, great amounts of steam are generated as energy is added during the refining operation on the fiberous material. This steam passes out of the refining space together with the refined material . . . PA1 Prior efforts to alleviate the problems associated with the generation of steam between the refining discs have involved withdrawing of steam from the central space between the refining discs. For example, Canadian Pat. No. 974,958, issued Sept. 23, 1975, for `Apparatus for Treatment of Cellulose Containing Material` discloses an apparatus and method in which steam generated during refining is withdrawn and discharged into the housing surrounding the refining discs through central openings close to the axis of rotation of the refining disc. That is, steam is withdrawn through openings arranged radially inward of the feed opening for the cellulose chips being introduced between the refining discs. While such arrangements have helped to relieve some of the problems associated with the generated steam, they have not been totally satisfactory, especially with respect to maintaining the stability of the refining gap and to assuring a uniform flow of material therethrough. PA1 In big grinders operated at considerable power levels, vaporization of the water contained in the wood produces so much steam that, because of the steam pressure, it is difficult to maintain a constant distance between the grinder cutters, which is important in view of product quality. Moreover, the steam bursting out of the grinder in an uncontrolled manner often involves significant trouble in the supply of material into the grinder. PA1 An advantageous embodiment of the invention is characterized in that the exhaust channels serve primarily to exhaust the steam produced in the grinding process, and that the velocity of the steam flowing in the exhaust channel depends on the sectional area of the channel, and that when the velocity of the exhaust steam is over 10 m/s or about 10-50 m/s, the defibrated material is drawn by the steam flow into the exhaust channel and thus removed from the space between the cutters. As the defibrated material is quickly removed by the steam flow, over-grinding of the fibres is prevented, while less energy is consumed in the process. PA1 The sectional area 9 of the exhaust channel 7 is considerably larger than that of a conventional groove 6 between the cutter teeth, which means that the steam is efficiently exhausted through the channel. If the exhaust channel 7 is made to correct dimensions so as to provide an appropriate passage for the amount of steam produced, the violent flow of steam carries the finest material, i.e. the fibers, along with it out of the grinder. PA1 Additional edge formations 12 or protrusions 13 at the bottom of the exhaust channel may also be incorporated to control the amount or kind of material that can be carried along by the steam flow. PA1 In order to maintain this material in the second refining zone 18 as long as possible, each groove 32 has at least one, and preferably two, dams 34. As shown in FIG. 3, these dams 34 are preferably surface dams (but could be subsurface dams), which means that the dams extend upwardly so that the top surface 36 is at the same elevation as the top surface of the adjacent bars 30. As described above, the dams 34 interrupt the flow of material through the grooves 32, forcing the material onto the adjacent bars for further refining. In the second refining zone 18, substantial quantities of steam are also generated, producing a steam flow with high radially outward velocity. PA1 As a result of the angular orientation of the bars 36 and grooves 38 in the outer refining zone 20, and the centrifugal forces acting on the steam and partially refined fibers, a natural separation of steam and fibers occurs in an advantageous manner. The steam, unimpeded by dams in the channels 38, flows relatively easily through the channels and exhausts at the outer edge 24. The fiber, being heavier, is thrown toward the trailing wall 46 of each groove 38 and is thereby forced onto the upper surface of the trailing bar 36, for additional refining action. PA1 In any event, the invention contemplates an inner zone or pattern of substantially radially oriented bars 30 and narrow inner grooves 32 having dams for interrupting the radial flow of material therethrough, and an outer zone 20 of outer bars 36 and wide outer grooves 38 defining flow channels 48 extending from the inner pattern 18 to the outer edge 24 of the plate at an angle of at least about 45 degrees relative to the inner grooves 32. The channels 48 extend from the grooves 32 of the inner zone 18, to the outer edge 24 of the plate, substantially in the direction of disc rotation 106, and have little or no dam structure for interrupting flow. PA1 Fluid-resonant oscillations mainly due to the radial slot profile provide the tuned resonant cavity mode for various sized wood chips . . . , and for slot resonance with internal dams forming a series of resonance cavities when filled with any combination of wood chips, wood fibers, water, steam or air. PA1 a high power input due to wasted energy with large fluid-dynamic drag, much noise, and considerable erosion loss with an untuned resonating cavity. PA1 The slots defined between the bars have a parallel radial profile 21, each slot having a horizontal bottom surface 21a. A multiplicity of dams 22, shown in section B--B, are located and spaced in each of the slots of the slot profile 21. The dams 22 are evenly spaced in each slot, but staggered in parallel slots, and at mid-line radial bar crossings, can produce a cellular standing wave that can cause a steam flow restriction called rotating stall.
Efforts to solve problems caused by the steam are discussed at column 1, beginning at line 41, as follows:
In an effort to prevent fiber being swept out of the refining zone by steam being vented, and thus decreasing the shredding action of the ribbed discs on the fibers in the refining zone, dams spaced radially of the ribbed discs have been formed in the grooves between the refining ribs so that the fibers cannot be swept without restraint along the grooves by the action of the steam being vented and the centrifugal force produced by rotation of one or both disks.
While such dams interrupt free flow of fiber radially along the disk grooves, the fibers tend to pack in the groove pockets formed between the dams so that much of the fiber simply orbits segregated from the fiber in the refining zone and thus reduces the shredding capacity of the refiner without corresponding improvement in the refining action.
At the same time, clogging of the grooves by dams between which fiber packs blocks the passage for escape of steam from the refining zone so that the pressure of the steam between the disks has increased, and the great steam pressure makes it difficult to maintain a constant distance between the refiner disks as stated in Perkola U.S. Pat. No. 4,676,440, issued Jun. 30, 1987, at column 1 lines 25 to 33:
In addition to larger friction forces, an increase in kinetic energy due to the clogged grooves will require more power to turn the disks.
To reduce the steam pressure between the disks by increasing the venting of the steam, it has been proposed to increase the cross-sectional area of the grooves between the refining ribs, but such expedient has simply increased the size of the pockets in which fibers can pack that merely orbit removed from the refining zone with a relatively small increase in the venting area.